US2516272A - Frequency conversion system - Google Patents

Description

July 25, 1950 M. L. THOMPSON FREQUENCY CONVERSION SYSTEM 2 Sheets-Sheet 1 Filed Dec. 7, 1945 IN V EN TOR. MM 70N 1. rHoMPso/v BY H/s HQE/vr m MW E July 25, 1950 M- THOMPSON 2,516,272

FREQUENCY CONVERSION SYSTEM Filed Dec. 7, 1945 2 Sheets-Sheet 2 INVENTOR.v /7/ 70A/ L. 7'HOMP50N Patented July 25, 1950 UNITED PATENT oFFlcl:

FREQUETNcYfeoNvERsioN"SYSTEM Milton-L. 'lthompsomli'untingdonfv alley, Pa., =as

signor, by mesne. assignments,` to Philco Cor-L, poratior,Plladelphia, Pa., al corporation of Pennsylvania Application` December 7, 1945; Serial No. 633,453l eciaims. (o1. 2501-) This invention relates!r to improvements. inl systems lfor, converting. modulated-` carrier` Wavesignals; tocarrier wave signals off substantially different-frequencies.- but having essentially the.v

same. modulationf as thel originalcarr-ier Wave signals. I-n l particular it A relates to. improvements in. such systemsY applicable to radio receivers adaptedrfor the reception ofsignals in the very high frequency range;r or1 for the reception of signals-in'` this andfinrlower frequencyl ranges. Specicallythese improvements' are especially applicable in multi-band systems-for the recepj tionlof bothzamplitudeand frequency-modulated signals; where,.. in accordance with present prac tice, amplitude-modulatedv signalsy are in; a rela'- tively low frequency range, while frequencymodulated4 signals: are situated in the. veryY high" frequencyfrange (88-108 megacycles);

'Ihe-rv application-y of.- the superheterodyne principle in the amplification ofibothamplitude-A andfrequencymwdulated carrier wave' signals isfwell known.v However-1,' in the application ofthisprinciple to signals in the very high frequency range, difficulty may be experienced owing to the frequency instabilityof local'oscil' latcrslused:togenerate-signals to be mixed,` with the; received signals` to yieldv anV intermediate frequency signalwhich.. is more convenientlyl amplified.` Such frequency instability is particularly objectionable when" the received i signal to'fbe' amplified isi frequency-modulatedv The: principal object-of this?" invention is to*-` provide means. for supplying ai local` oscillator signal. ofhigh frequency stability to the frequencyf converterA tube of a superheterodyne radio receiver'adapted for usein thel reception of signals in the very high frequency range:

Another object of the` invention is toprovide,

in a: superheterodyne radio receiver adapted to y the reception of signals in the. very high frequencyrangaas well as lower` ranges, asource:

of-ay signalof inherently high frequency stability fon-mixingY with the received-very high frequency signal to produce an intermediate frequency signal, saidsourceibeingseparate from and substantially independentof themeans used tolprovide-local signals for' mixingl with the receivedsignals' of-` lower frequency, and therefore free from certain disturbances which wouldfadversely aifectl its frequency stability.

Other objects' and featuresvofA the invention will be apparent from the following. description` and"l accompanying-v` drawings vin :which:

Figurel is aschematic diagram of a` portion of aradio4 receiver embodying one. form ofv the invention, in which 'a commercialY type 6.16 tube` is Yused as anloscillator-and. frequency multiplier on thel very high frequency Vv(frequency modulation) band-while atype 7Q7 is used asa free quencyconverter on the frequencymodulation (F'M band andas alocal oscillator-converter on otherfandlower frequency. bands. Although, foril purposes of simplicityein/ explanation, vonly one low frequency band lis providedv for in the circuit.

illustrated, it lis to be understood that additional 5 bands could be provided.- for in", a mannerl whichl is familiar to those-skilledinthe art; and M vFigure A-2 v is a schematic 1 diagramof. a variation of the portionrof the. circuit showntothe right of A--Af in .'Figure 1 embodying ,a somewhat dif# ferent form off the invention, in` which a1 type- '7E5tube isused. as; a converter to obtain lower input capacity andvihigher inputyresistance and in ywhich one section of Vaf typev 'TF8 tube, whenY not functioning as a frequency multiplier and.-l

buffer stage, isused as ar'local oscillator in the reception ofamplitude-modulated signals in the.

lower frequencyV ranges. Y

Referring now tofFigure 1, to '.thefleft. of line Ay-A! `are Vshown circuits yfor .the reception andradio.` frequency' amplication of -modul'ated carrier'vwave signals infveryhighffrequency andfin These signals may; yas v lower frequency ranges. has already been: suggested, be respectively fre quencyemodulated andfamplitude-modulated sig` nalsfin accordance with-lthe-present practice--in thisv country? of using.V carrierfrequencies inrthe` very high y frequency range for frequency modu-j-vv lated' transmission,"andcarrier yfrequencies in`v relatively lower frequencyrangesfor amplitude modulated#v transmission;A In" Aviewfyofr this practice; and: in view offthe fact that the-specific" embodimentsV of the invention here `'disclosed aref` particularly' adapted for the reception" of bothiamplitudeand." frequency-modulated signal-s;

the termsy frequencymodulated signa-l and ampli-L tude modulated signalfinayberegarcled-as*denot-g` ing,Y modulated carrieriwave signalsin theVVA very y high:y frequency range-and in`1owerL frequency In anyeventz theV use of` ranges respectively. these terms r in generalshouldnot; be= regardedy as limitingfv the.` inventionA to ruse rwith .signals of any;`

particular yform or type of modulation.,-

V1. With switches S1 and S2 both in their alternative positions, amplitude-modulated signals intercepted by the loop antenna 3 will be supplied to the control grid of the radio frequency amplifier, V1, through coupling means 4. Automatic volume control voltage, derived in a conventional manner ,atV a subsequent stage of the receiver (not shown), may also, if desired, be applied vto the control grid of tube V1 through the grid leak resistor 5 in the manner shown.

Depending on the positions of the switches S1, S2 and S3, either frequency-modulated signals will be supplied through the blocking condenser E to the circuit I which is variably tunable to signals in the FM range, or amplitude-modulated signals will be supplied to the circuit 8 which is variably tunable over the AM range. Signal voltages developed across either of these tunable circuits are supplied through blocking condenser 9 to a control grid of a pentagrid converterl tube V2.

With'switches S1 through S1 positioned for FM reception as shown in the schematic of Figure 1, local4 oscillator signal is supplied from a special frequency multiplier arrangement operating in the following manner. An oscillator, comprising a tuned circuit III loosely coupled between the plate and grid of the first section of the tube Va through the secondary winding of the transformer I I, generates a signal at a frequency corresponding to the third subharmonic of the signal desired for mixing with the frequencymodulated carrier wave signal to produce an intermediate frequency signal suitable for convenient amplification. If, for example, the desired intermediate frequency is 9.1 megacycles and the FM band to be covered is from 88 to 108 megacycles, the circuit I might be made tunable by means of the variable condenser C from 26 to 33 megacycles allowing for some overlap at y third harmonic of the oscillator frequency (i. e. Y

'78 to 99-megacycles). This tube section acts, therefore as a frequency multiplier and buffer stage. The signal developed across this tuned circuit is fed to a grid of the converter tube V2, which may be a commercial type 7Q7, and is thereby injected into the tube where it is mixed withvthe incoming frequency-modulated signal applied to the control grid to produce in the output circuit of tube V2 a frequency modulated intermediate frequency signal which is suppliedV through the upper section of the coupling transformer I3 to an intermediate frequency amplifier. This amplifier may be of conventional design as may the remaining receiver circuits, which are neither shown nor described, and may be adapted for the amplification of either FM- or AM signals as desired.

According to the invention, the oscillator just described is used only to supply the local oscillator signal for use in converting received signal Afrequencies in the FM band. Separate vmeans are provided to generate the local oscillator signal used in converting received signals in lower frequency ranges andk thereby a number` of factors adversely aifecting the frequency stabilityl of the FM oscillator are eliminated. In the embodiment of `Figure 1 the converter tube V2 is made to serve also as a source of local oscillator signal for the frequency conversion of 'signals in the AM or lower frequency range. To this end, with the switches S1 and S5 in the positions alternative to those shown, the tunable circuit I4 is connected in circuit with the cathode and first two grids of thetube V2 to form an oscillatory circuit, the signal from which is injected directly into the tube V2 for mixing with the incoming AM signal. Assuming that the AM band to be received extends from 550 to 1600 kilocycles (the standard broadcast band), and that an intermediate frequency of 455 kilocycles is desired, the AM oscillator may be made variable from 1005 to 2055 kilocycles by varying the capacity of the condenser Cv. Likewise, for the reception of signals in other AM bands, switchingmeans might be provided, adapted to alter the xed parameters of circuit I4 so as to make the frequency of the oscillator variable over `-other ranges and to yield the same intermediate `frequency or a different one. For the reception of AM signals switches Se and Sv are placed in the positions alternative tothose shown, and intermediate frequency signals derived from the output circuit of tube V2 are ysupplied through the lower section of the tuned -transformer I3 to the intermediate frequency amplifier and subsequent conventional receiver circuits.

As indicated in the diagram of Figure 1, A. V. C. voltage may be applied to the input grid of `theconverter tube V2 as well as to the radio frequency amplifier tube'V1 and other subsequent stages not shown. It is significant and a feature of the invention that this may be done Without adversely affecting the frequency stability of the FM oscillator because 'the latter is effectively modulatedsignals in the very high frequency range'areas follows:

(1) The higher tuned impedance obtainable in y the oscillator tank circuit I0 because of the lower frequency at which the oscillator is operated, together withthe choice of a tube of low capacity and high-mutual conductance, permits loose coupling of theftube to the oscillator tank circuit whichvgreatly reduces thev effect of tube interelectrode capacities on the oscillator frequency. (2) The use of Ya lower voscillator frequency and the consequent higher circuit inductance minimizes the eifect of,l lead inductance lon the oscillator frequency.v y t (3)y The use of theoscillator only in connection with the reception of signals inthe FM band, y

and the consequent yeliminatic'in of switching in the oscillator circuit, greatly reduces the amount of unstable inductance and capacity in the oscillai tor circuit and thereby greatlyy improves its stability.

(4)- Since the oscillator is not directly coupled quency of the oscillator.

(6) The greater amplification' provided by the)V useof thetuned buier stage permits the operation of the oscillator at a lowersub-harmonic of control may be applied to the converter tube without'aifecting the fre- ?:'11 the frequency'-desired` for mixing-'purposes than would y otherwise bei possible I while maintaining the' translation gain ofthe frequency converter! possible` vgreater oscillatorl stathereby making bility. f

Reference is now made to; Figure 2, which represents-a variation of the circuits to the right oflineA--AJ in Figure l inaccordance with ari'- other embodimentY of the invention.r With. switch S8 in the position shown,k frequency modulated signals'` supplied Vfrom conventional circuits to the; leftof the line A -A th'rOll-gh the blocking condenser I5; will be developed across thelzcircuit I6 which tunable by I rieansjgof;` the variable ccnpdexlserTv C81 toY frequencies. im. the FM.; range: 'Ehesesisnalsle supplied-.threugh a blocking condenser l1 to the grid of tube V4, which may be a commercial type 7E5, used in this embodiment of the invention, because of its lower input capacity and higher input resistance, to provide greater gain than can be obtained from the 7Q7 used in the embodiment of Figure 1. Local oscillator signal for effecting the frequency conversion of the frequency modulated signal is obtained from an oscillator-frequency multiplier arrangement similar to that shown in the first embodiment, except that in this instance the oscillator comprising the variably tunable circuit I8 and the first section of the tube V5, which may be a commercial type 7F23, is operated at the second subharmonic of the desired local oscillator signal. This is done to obtain a higher signal voltage for injection into the cathode circuit of tube V4. The signal generated by this oscillator is fed directly to the grid of the second section of tube V5, from the tunable circuit i9 in the output of which, signal is fed to the cathode of V4. The load impedance in the output circuit of this section may include, in addition to the resistor 25, an inductor 21 adapted to raise the level of the second harmonic output. The circuit I9 may be tunable by means of the variable condenser Cn to frequencies in the range from 78 to 99 megacycles to provide coverage of the 88-108 megacycle FM band with an intermediate frequency of 9.1 megacycles. For FM reception switches S8 through S14 should be in the positions shown. In this instance intermediate frequency signal will appear in the output circuit of tube V4 and will be supplied through the upper section of the tuned coupling transformer 20 to the intermediate frequency amplifier and subsequent conventional receiver circuits (not shown).

For the reception of AM signals switches Ss through S14 should be in the positions alternative to those shown. Then AM signals developed across circuit 2|, tunable by means of variable condenser C9 over the AM band, Will be supplied to the input grid of V4. In this instance, local oscillator voltage is generated in an oscillator comprising the second section of tube V5 and the circuit 22 tunable by means of the variable condenser Ciz over the desired range of local oscillator frequencies. The plate and cathode of this tube section are connected to taps on inductor 28 of the tunable circuit in the manner shown.

The grid is effectively grounded at these fre-f. quencies through the blocking condenser 23 and Va is-.supplieda through ythe lowerxsectiorr of the? tunedlcoupling; transformer 20r tothe` intermediate frequency amplifier anch subsequent circuits.

be l. ents illustrated-iinthel drawings, it vvillvbe4 understood that-the invention is generally applicable-r toradio receivers for-the receptiontgoff4 suchsignalsl in theffvery, high frequency range andg inrlower ranges, and adaptedr-to other forms of physical.expression,I and is therefore-not :to Vbe.

limited tothespeciiic Vdisclosure.`

1.; lar-15av superheterodynevradio receiver for selectively receiving modulated carrier Wave signals in dierent frequency ranges; a frequency converted stage comprising a first electron discharge device having plural electrodes and being operative in response to incoming modulated carrier wave signals and local oscillations supplied respectively to different ones of said electrodes to convert said incoming signals to an intermediate carrier frequency; a source of local oscillations suitable for effecting conversion of modulated carrier wave signals within one of said ranges, said source comprising an oscillator and a harmonic generator circuit supplied with oscillations from said oscillator and operative to generate local oscillations which are harmonically related to the oscillations produced by said oscillator, said generator incorporating a second electron discharge device; resonant circuit means cooperative with one of said electron discharge devices to provide a second oscillator constituting a source of local oscillations suitable for effecting conversion of modulated carrier wave signals within the other of said ranges; and means for selectively rendering said sources effective to supply local oscillations to said converter stage.

2. In a superheterodyne radio receiver for selectively receiving modulated carrier Wave signals in different frequency ranges; a frequency converter stage comprising a first electron discharge device having plural electrodes and being operative in response to incoming modulated carrier wave signals and local oscillations supplied respectively to different ones of said electrodes tc convert said signals lto anintermediate carrier frequency; a source of local oscillations suitable for effecting conversion of modulated carrier wave signals Within one of said ranges, said source comprising an oscillator incorporating a second electron discharge device; resonant circuit means cooperative with said first electron discharge device to provide a second oscillator constituting a source of local oscillations suitable for effecting conversion of modulated carrier wave signals Within the other of said ranges; and means for selectively rendering said sources effective to supply local oscillations to said converter stage.

3. In a superheterodyne radio receiver for selectively receiving modulated carrier Wave signals in different frequency ranges; a frequency converter stage comprising a first electron discharge device having plural electrodes and being operative in response to incoming modulated carrier wave signals and local oscillations supplied respectively to different ones of said electrodes to convert said incoming signals to an intermediate carrier frequency; a source of local oscillations suitable for effecting conversion of modulated carrier wave signals within one of said ranges, said source comprising an oscillator cir-- 7 j.. cuit, incorporating a second electron discharge device, and a harmonic generator circuit supplied with oscillations fromsaid oscillator and operative to generate local oscillations which are harmonically related to the oscillations produced by said` oscillator, said generator incorporating a third electron discharge device; a source of local oscillations suitable for supply to said converter stage'to eect conversion ofmodulated carrier wave signals Within the other of said ranges, said source comprising a second oscillator circuit incorporating said third electron discharge device, and means for selectively `applying local oscillations produced by either of said sources to said converted stage.

' ,MILTONL THOMPSON.

s Number .fo-REFERENCES CITED The following references' are of record in the flle of this patent.:

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